This invention relates to transmissions for vehicles in general, and more particularly to hydrostatic transmissions for vehicles, such as, for example, garden tractors and lawn tractors. A hydrostatic transmission (HST) is a well known type of continuously variable transmission wherein a system of one or more hydraulic pumps transmits energy from an input shaft to one or more hydraulic motors connected to one or more output shafts. The input shaft is typically driven by a prime mover, such as an internal combustion engine or electric motor. The input shaft may be driven directly by the prime mover or indirectly, such as by a belt and pulley system connected to the prime mover. The one or more output shafts typically are connected to the vehicle's rear wheels to propel the vehicle. The hydraulic pumps and motors are contained within a transmission housing filled with hydraulic fluid, which flows in a fluid circuit from pump to motor and back again through internal porting and passages. It is this flow of hydraulic fluid which transmits energy from pump(s) to motor(s). Direction of vehicle travel is dependent on the direction of flow within the hydraulic circuit.
Various HST configurations have been used on smaller vehicles, such as lawn tractors, to overcome vehicle layout constraints. For instance, U.S. Pat. No. 6,941,753 discloses an HST transaxle having a single pump and single motor in hydraulic communication via a center section. U.S. Pat. No. 7,229,256 discloses a different HST design utilizing two separate pumps, both contained within a single housing. Still another configuration is disclosed in U.S. Pat. No. 7,225,617, wherein two separate transmission housings are employed, each containing a separate hydraulic pump/motor circuit to drive wheels independently. In configuring an HST, allowances must be made for wheel radius and width, auxiliary implements such as a mowing deck, and other essential components, all while maintaining appropriate ground clearance. Some of these constraints are discussed in U.S. Pat. No. 6,986,406, which introduces a two-piece housing shaped to allow for rear discharge of cut vegetation to a rear receptacle. The teachings of the above-referenced patents are all incorporated herein by reference. When the wheels being driven by the output shafts also have a steering function, space to package an HST may be even more limited and trade-offs may result, such as more intricate and costly HST designs or a limited turn radius.